1. Technical Field
The present invention is directed toward rotating electric machines having a slip ring.
2. Description of the Related Art
A conventional alternator 10 is illustrated in FIG. 1 sometimes referred to herein as a generator. Alternator 10 has a rotor assembly generally designated by the reference numeral 20 and stator assembly generally designated by the reference numeral 15. The rotor assembly 20 includes a shaft 21 supporting all rotating magnetic circuit structures thereof including conventional pole-members 16A and 16B, rotor core 17 and field coil 18 wound upon bobbin 12. Additionally, all other non-magnetic circuit rotating structures are carried thereby, including air circulation fans 19 and 27 located at axially opposite sides of the pole-members, and a slip ring assembly 30 located at one extreme end of the shaft. Fan 27 is formed from sheet metal stock and spot welded to pole-member 16B while fan 19 is formed from an appropriate thermoplastic material and is ultrasonically welded to tower extensions (not shown) from the field coil bobbin 12. The shaft 21 in turn is rotatably supported within a housing 26 by a pair of bearings 23 and 22. Bearing 23 is located between the slip ring assembly 30 and the fan 19.
Coil leads 18A of field coil 18 are wrapped about respective posts 12A of bobbin 12 and pass through holes 13 in fan 19. Slip ring assembly 30 is made of a pair of copper rings 31, each having a slip ring lead 32 joined such as by welding thereto. The copper rings and wires are molded into a thermoset material to complete the slip ring assembly. A one-piece slip ring assembly is disclosed in U.S. Pat. No. 4,961,016 issued to Gold. Slip ring assembly 30 is pressed onto the end of rotor shaft 21 and the slip ring leads 32 are routed into channels along the shaft 21 where they are joined, such as by twisting and welding, to the coil leads 18A of field coil 18 via a joint 24. The joint 24 is then bent to the surface of the fan 19 and received in a pyramid-shaped tab structure 25. The joint 24 is then secured to fan 19 by ultrasonic welding of the plastic material of the tab 25. Bearing 23 is assembled to pass over the slip ring assembly 30 to retain the lead wires 32 securely within the shaft channels. The configuration in FIG. 1, however, presents several manufacturing challenges which may affect long-term durability.
First, the weld connection at joint 24 may be made imperfectly, for example, where the weld bead contains a partial crack or fissure. Such a joint will normally pass electrical tests conducted during manufacture. However, during the service life of generator 10, the rotational forces (i.e., centrifugal forces) that come to bear on joint 24, either directly or indirectly (e.g., via flexure of the fan body) may cause the crack to propagate until the electrical connection is broken entirely, resulting in an open condition in the field winding circuit. This will cause the generator to fail.
Second, the above-mentioned ultrasonic welding operation may incompletely melt the plastic in the vicinity of the joint 24, resulting in gaps or voids. These gaps or voids may allow the wires or joint 24 itself to move during operation of the generator (i.e., rotation of the rotor). This movement may fatigue the metal, causing it to break, resulting in an open circuit and failure of the generator. Additionally, a weld horn that is used in the ultrasonic welding operation may contact the wires or joint 24 directly (not just the tab 25). The high vibration imparted by the weld horn may fatigue the wires or joint 24, perhaps not severe enough to cause a break that could be detected during manufacture via electrical testing. Moreover, the weld horn may crush, flatten or otherwise deform the wires or joint, thereby weakening it. The wires/joint, thus weakened, may fail during operation of the generator.
Third, slip ring conductor 32 may be pulled too tight when joint 24 is made, causing it to rise slightly out of the channel in the shaft and fan hub where it is designed to reside. Bearing 23, when assembled onto shaft 21, may contact conductor 32, deforming or possibly cutting the wire""s outer insulation. Such a condition will ground the rotor winding, causing the generator to fail.
Fourth, the connection of conductors 32 to respective slip rings 31 is conventionally made via brazing or welding. As shown in FIG. 1, the slip ring connections are located near a radially outermost portion of the slip ring assembly 30 itself. Rotational forces increase with increases in distance from the main axis, and may therefore weaken such connections, causing some to fail.
U.S. Pat. No. 5,625,244 to Bradfield discloses a slip ring assembly having channels for capturing coil leads and routing them through the slip ring assembly to an axially outermost end. The channels, however, are disclosed as being of uniform size, allowing an insulating sleeve associated with the coil leads to emerge from the slip ring assembly and possibly interfere with an electrical connection. In addition, the channels route the coil leads near a radially outermost portion of the slip ring assembly, subjecting the leads/connections to an increased centrifugal force, increasing the chance of a failed electrical connection.
There is therefore a need for an improved generator that minimizes or eliminates one or more of the problems as set forth above.
An object of the present invention is to provide a solution to one or more of the problems set forth in the Background. An advantage of a slip ring assembly in accordance with the present invention is that it reduces the number of electrical connections compared to conventional designs. This reduced number of connections reduces manufacturing complexity and improves durability. Another advantage is that it provides a more robust mechanical and electrical slip ring connection, inasmuch as it is subjected to reduced centrifugal forces by being located nearer the main axis of the slip ring assembly. Yet another advantage is that interference of an insulating sleeve associated with the rotor coil leads is reduced, via routing through-bores that have a reduced diameter exit aperture, preventing the sleeve from exiting.
These and other features and advantages are achieved by a slip ring assembly in accordance with the present invention. A slip ring assembly is provided for supplying current to a rotor winding of a rotor of an alternating current generator and includes a body portion having a main axis formed of electrical insulating material, the insulator body having a central bore configured to be secured to a rotatable rotor shaft, the body portion having first and second opposing ends, the first end configured to face the rotor, a pair of spaced electrically conductive slip rings engaging the body portion, each slip ring having a respective coupling terminal extending from the second end of the insulator body portion, characterized in that the body portion has a pair of longitudinally extending through-bores configured to allow rotor winding leads to pass from the first end to the second end.